Hydrogels, which may be defined as macromolecular networks swollen in water or biological fluids, are known for various biomedical applications.
Further, hydrogels exhibiting the specific property of having their viscosity increasing when the temperature increased, also called “thermosensitive hydrogels”, were proved to have a facilitated application combined with an increased residence time at the site of application and therefore were found advantageous as drug delivery system.
As known from O. Felt et al. in The Encyclopedia of Controlled Drug Delivery, 1999, said thermosensitive hydrogels may be based advantageously on polymers of natural origin, for example on chitosan which is a commercially available inexpensive polymer derived from chitin, the second most abundant polysaccharide after cellulose.
Chitosan is known as a chitin derivative obtained by partial to substantial alkaline N-deacetylation of chitin also named poly(N-acetyl-D-glucosamine), which is a naturally occurring biopolymer, found in hard shells of marine living animals such as fishes, crustaceous, shrimps, crabs, etc., or synthesized by natural organisms such as zygomycete, fungi, etc.
Chitosan contains free amine (—NH2) groups and may be characterized as to the proportion of N-acetyl-D-glucosamine units and D-glucosamine units, and such is expressed as the degree of deacetylation (DD) of the fully acetylated polymer chitin.
Parameters of chitosan influencing important properties such as solubility and viscosity are the degree of deacetylation (DD) which may be understood as representing the percentage of deacetylated monomers, and the molecular weight (MW).
Chitosan is known to be biodegradable, biocompatible, bioadhesive, bacteriostatic, and further to promote wound-healing, drug absorption, and tissue reconstruction.
Due to its above mentioned intrinsic properties, chitosan is known to have numerous cosmetic and pharmaceutical activities, and has been also widely explored for various applications through gels.
Therefore, considering the advantageous properties of chitosan, there is a continuous need to improve the properties of known thermosensitive chitosan hydrogels which are still considered as very promising for a wider range of biomedical applications.
WO-A-99/07416 (Biosynthec) discloses a pH-dependent temperature-controlled chitosan hydrogel which has thermosensitive properties at neutral pH such that it has low viscosity in the cold but gels at body temperature.
This thermosensitive chitosan hydrogel is prepared by neutralizing a commercial chitosan having a deacetylation degree of about 80% with mono-phosphate dibasic salts of polyols or sugars exemplified in particular by β-glycerophosphate (β-GP).
Addition of β-GP to chitosan allows to increase the pH up to 7 without chitosan precipitation and to form a hydrogel on a temperature dependant way, i.e. the higher is the temperature, the faster is the gelation process.
Said hydrogels are advantageous in that they contain biocompatible components and a high percentage of water, in that they have a physiological pH and in that no heat nor product is released during gelation.
As also reported by A. Chenite et al. in Carbohydr. Polym. 46, 39-47 (2001), in relation with chitosan/β-GP pseudo-thermosetting hydrogels, adding β-GP for neutralizing high DD chitosan which are known to precipitate above pH 6.2 allows to prevent precipitation of said high DD chitosan.
However, presence of β-GP in the hydrogel leads to the following disadvantages.
β-GP is a negatively charged entity that can react with a positively charged bioactive component, leading to its precipitation or to the disturbance of its liberation from the hydrogel.
Therefore, presence of β-GP renders chitosan/β-GP hydrogels inappropriate for use with numerous drugs.
Further, the modulation of the properties of this hydrogel, such as gelation time and viscosity, depends on the concentration of β-GP and is therefore limited by the solubility of β-GP.
In particular, a high concentration of β-GP is required to have a low gelation time avoiding the rapid elimination of the hydrogel after its administration.
However, a high concentration of β-GP also decreases the viscosity of the hydrogel.
Therefore, the gelation time has to be balanced with the consistency of the hydrogel, and it is not possible to obtain gels that have both a low gelation time and a high viscosity, which would be a desirable combination of characteristics.
Also, a too high concentration of β-GP may induce the precipitation of the hydrogel at its administration site.
Further, said thermosensitive chitosan/β-GP hydrogels were found to be turbid, thus rendering their use inappropriate for particular applications such as ocular or topic administrations.
On the basis of these facts, the present inventors have continued their researches to overcome the disadvantages of the known thermosensitive chitosan/β-GP hydrogels and have surprisingly found that by using a reacetylated chitosan having a deacetylation degree of 30-60%, neutralization of chitosan to form thermosensitive hydrogel may be made by addition of NaOH or any other hydroxylated base instead of β-glycero-phosphate, and further that if reacetylation of chitosan to DD 30-60% is made in homogeneous conditions, a transparent chitosan hydrogel is obtained.
The present invention has been achieved on the basis of these results.